Automatic gain control (AGC) is widely used for handling varying received signal strengths in wireless receivers. A typical AGC control system measures the signal strength of the received signal, e.g., by analyzing a digitized data stream, and compares the measured signal level to a desired signal strength, a so-called set point. The difference between the measured signal strength and the desired signal level is used to control one or several amplifiers, to keep the output signal as constant as possible with respect to signal strength, or within a desired range, at a given reference point in the receive path. In many systems, the signal strength is processed in logarithmic units, so that the gain control system has a linear response with respect to decibels.
In a digital wireless system, the received signal is initially an analog signal that must be converted into digital form to extract the encoded digital information. The analog-to-digital (A/D) converter used for this conversion has limits on its dynamic range, and often has a dynamic range that is far less than the desired system dynamic range for the receiver. This means that the analog gain of the receiver path must be regulated, based on the received signal strength, before the signal is converted to digital form. However, conventional techniques for regulating the analog gain of the system can give rise to undesired transient effects. For instance, each analog gain adjustment typically results in a change in DC offset in the digitized signal, in addition to the gain change. This unwanted effect can distort the signal and cause degraded performance. Although these DC offset changes can be mitigated with high pass filtering, frequent analog gain changes will still cause degradation of the signal, since it takes a while to remove the offset of each gain change by this filtering.
One way to minimize such transient effects is to make as much use of the dynamic range at the A/D converter as possible, thus adjusting the analog gain prior to the converter only when necessary, e.g., only when the received signal strength passes outside some pre-defined boundaries. Remaining gain adjustments, e.g., to provide a level signal for digital demodulation, may then be performed in the digital domain, e.g., using a digital gain amplifier (DGA). These digital gain changes do not give rise to distorting transient effects.
In a multi-carrier system, two or more distinct radio-frequency carriers are placed close to one another in frequency, with both carrying information targeted to a given receiver. As specified by the 3rd-Generation Partnership Project (3GPP) for Wideband Code-Division Multiple Access (W-CDMA) systems, a W-CDMA multi-carrier system in its simplest form uses two adjacent carriers of width 3.84 MHz, with approximately 5 MHz spacing between the carrier center frequencies. However, the 3GPP standards also support use of different spacing and different numbers of carriers.
Regardless of the details of carrier spacing and bandwidth, each carrier will be subject to different variations in received signal strength as received through the antenna or antennas of a wireless receiver, even if the carriers are transmitted with identical signal levels. Thus, one approach to automatic gain control in a multi-carrier receiver is to use two independent receiver chains, each with a separate and independent AGC system. The independent AGC systems provide each carrier with the gain changes needed to keep that signal constant in signal strength. However, this approach can be very expensive in terms of receiver components and integrated circuit chip area, which translate directly to production cost for the receiver. Furthermore, to the extent that independent receiver chains are used for each carrier, the power consumption of the receiver will tend to increase linearly with the number of carriers. Since market pressures make it very important to keep the number of radio parts and the power consumption as low as possible, multi-carrier receiver designs that re-use several components of the receiver are highly desirable.